For the past decade, monitoring electrostatic charge in the exhaust gas of a jet engine has been investigated as a possible indication of engine deterioration and/or impending engine failure. As reported in U.S. Pat. No. 3,775,763, it was originally thought that small particles of engine parts, having a net charge, caused spikes of ion current of relatively large magnitude when striking an electrostatic probe. Later, it was theorized that the electrical activity on ionic probes was the result of Trichel pulses (a form of repetitive corona discharge) created by high potential pockets of excess charge, related to engine wear. This is reported by Couch, R. P.: "Detecting Abnormal Turbine Engine Deterioration using Electrostatic Methods", Journal of Aircraft, Vol. 15, October 1978, pp 692-695. The apparatus described in said article utilized a probe set including circular insulated segments within the gas turbine engine tail pipe and a triangle of wire extending through the tail pipe exhaust gas path. With these probes, a normalized count of probe current (or voltage developed across an impedance) in excess of a threshold magnitude, over a period of time, was found to definitely correlate to impending engine component malfunctions or severe deterioration. However, the use of normalized counts of large magnitude signals from the ring and grid probe was found to provide reliable prediction of only two out of three gas path failures, at best, and was found to not provide any distinction between possible causes thereof.
In this prior period of time, attempts were made to determine whether or not there were any electrostatic phenomenon observable in other parts of a gas turbine engine which would be of any significance in engine diagnostics. However, no useful signals were found.
Obviously, the usefulness of any diagnostic tool is generally enhanced by the variety and versatility thereof.